Semiconductor pressure transducers are frequently used in applications which require operation in harsh environments that are corrosive and/or involve high temperatures. Accordingly, the stress sensing network of transducers used in such applications, must be protected from these harsh environmental conditions in some way in order for the transducer to remain operational over extended periods of time. In the past, surface over-coatings such as silicon nitride, silicon dioxide and the like, have been provided over the stress sensing network to protect it from harsh operating environments. However, such coatings provide only partial protection as the metallized contact areas of the sensing network remain exposed, which presents problems in applications which involve corrosive environments.
One method which enables the transducer to remain operational in corrosive and high temperature environments involves exposing the backside of the transducer to ambient pressure while hermetically sealing the stress sensing network located on the front side thereof, in the active portion of the diaphragm surface. Such a method is described in co-pending U.S. patent application Ser. No. 08/458,405 filed on Jun. 2, 1995 entitled: HERMETICALLY SEALED TRANSDUCER AND METHODS FOR PRODUCING THE SAME, by A. D. Kurtz and assigned to Kulite Semiconductors Products, the assignee herein. The co-pending application describes a hermetic seal which utilizes a PYROCERAM glass frit or similar means, to bond a cover member to the front side of the transducer to hermetically seal the sensing network.
The glass is used in the above-described application to bond the cover member to the inactive portion of the diaphragm because the fabrication processes used in forming the sensing network of the transducer will result in raised surface features when the transducer has a silicon-on-oxide (silicon-on-silicon) structure. The raised surface features such as where the sensing network extends into the contact areas, are not planar with the underlying substrate since they are formed on top of the oxide. This makes it difficult to form a hermetic seal by electrostatically or fusion bonding a cover member to the inactive portion of the diaphragm. For instance, in a silicon-on-oxide transducer structure, the entire silicon network and particularly, the regions on the inactive portion of the diaphragm leading to the metalized contact areas (lead-outs) are elevated from the oxide by several microns. Moreover, a finite space is provided between these raised lead-outs on the order of mils to prevent them from electrically shorting together. The step height of the lead-outs and the finite spaces therebetween create gaps between the cover member and the inactive portion of the diaphragm. These gaps make it virtually impossible to form a hermetic seal between the cover and the inactive portion of the diaphragm by electrostatic or fusion bonding and thus, a glass frit can be used to fill the gaps to create a hermetic cover.
Although the glass frit enables the cover member to be bonded to the inactive portion of the diaphragm to hermetically seal the stress sensing network, there are problems associated with this method. For example, the glass frit has a tendency to flow onto the diaphragm during the high temperature sealing process which causes the performance of the sensors to degrade. There are also thermal mismatching issues and other process compatibility issues which require consideration when using a glass frit. All of this makes the glass frit a difficult material to use for fabricating hermetically sealed covers.
Another problem associated with such a sensor or transducer structure is that the ball bonded lead wires are exposed to the operating environment. Consequently, particles which may be present in the pressure transmitting environment can come into contact with the lead wires and possibly damage them.
During the last few years considerable time and research has been devoted to the development of dielectrically isolated silicon on silicon piezoresistive sensors. In prior art devices the resulting structure consists of a highly doped p+ sensor network fusion bonded to an oxide layer which has been grown on an underlying support wafer. The support wafer in turn forms the flexing portion of the sensor. For instance, a dielectrically isolated Wheatstone bridge sensor consists of four resistor elements situated in a strain sensitive area of the sensor as well as at least four lead-out portions which are directed to contact areas on the non-flexing portion of the structure. It is of course obvious that each of the lead-out structures must be isolated from each other and there must be a space between them. When the sensor is affixed to an additional support structure, on the side of the sensor away from the sensor-resistive network, obtaining an hermetic seal between the sensor and the additional support structure is no problem because the non flexing surface of the support wafer is planar.
However when one attempts to fasten a support member on the sensor-network side, several problems emerge. Although it is possible to seal to the emerging fingers from the sensing network, the spaces between the emerging fingers extend into the flexing region and no hermetic seal can result. This is a particular problem when ones does not want to expose the sensing network side of the sensor to a specific pressure medium. For this case the side of the sensor not exposed to the media must be bonded to the support structure. However, this means that the bond must be made to the side containing the sensor network and the contact fingers. Thus the air leakage paths along the fingers must be sealed. One obvious method of obtaining a seal to attach the support structure to the network side of the sensor is to use a thick enough glass frit which will fill in the spaces between the fingers as well as seal the non deflecting portion of the sensor surface to the support. However, this method gives rise to certain problems. The frit generally has a different and higher thermal expansion coefficient then the silicon sensor structure and the support. It will, therefore, introduce unwanted thermal stresses in the resulting final structure. In addition, during deflection, mechanical stress is applied to the frit and for large stresses at high temperature it may yield. Moreover, the frit to structures seal is made at high temperature and some of the frit may flow onto the flexing portion of the sensor or into the contact areas thus degrading the device performance.
It is, therefore, an object of the present invention to provide an improved method for hermetically sealing a semiconductor pressure transducer which substantially eliminates the use of a glass frit to bond a cover member to the active side of the transducer.